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Measurements of low 14C activities by liquid scintillation counter at Zagreb Radiocarbon Laboratory

Recently we obtained a new liquid scintillation counter (LSC) Quantulus 1220, which will improve the capability of low-level 14C measurement in our Laboratory, where the gas proportional counter (GPC) has been used for more than 30 years. Two methods for chemical preparation of 14C samples are implemented: a CO2 absorption method (LSC-A) and a benzene synthesis method (LSC-B). CO2 obtained by combustion of an organic sample or by dissolving a carbonate sample in hydrochloric acid is used in both methods. The vacuum line for CO2 absorption consists of a container with CO2 sample, a U-tube with silicagel for CO2 purification, a manometer and a needle valve for CO2 flow regulation, a 20 mL glass vial with absorption mixture (Carbosorb E + Permafluor E) cooled with ice and flow meter (bubbler). Tests with different ratios of absorption mixture components and with different flow-rates of CO2 were performed to obtain optimal conditions of sample preparation. For LSC-B method the vacuum line for benzene synthesis has been constructed. The first step involves the reaction of CO2 with lithium and the subsequent hydrolysis of lithium carbide to acetylene. The reaction goes on in a stainless-steal vessel with heating and cooling systems, and pressure control, as well as with the window for the reaction control. Trimerisation of acetylene to benzene is performed on the Arslanov's vanadium catalyst. Several tests for benzene synthesis were performed: duration of carbide and hydrolysis reaction, type of C2H2 purification, duration and temperature of catalytic trimerisation. The acetylene yields range from 89 to 98%, and benzene yields range from 77 to 90%. The organic purity of benzene is 98.9 to 99.5%. Benzene was put into 7 ml glass vials and 15 mg of butyl-PBD per 1 g of benzene was added. For samples prepared by both LSC-A and LSC-B methods the optimal counting windows for measurement in LSC were determined. Samples were counted in 20-30 cycles of 30 minutes. The total efficiency of LSC A method is 65% and of LSC-B 85%, while the corresponding figure of merits are 18 and 336. The quenching correction curves for several counting geometries (2, 3 and 5 mL of benzene) were determined. 14C activities measured by LSC-A and LSC-B methods were compared with those measured by the GPC method (efficiency 56%, figure of merit 76). The results obtained by the LSC-A method have larger error than those measured by the GPC method, but the method is quick, cheep and simple and requires less carbon (~0.5 g) than the GPC method (~4 g). It is suitable for 14C measurements of geological, hydrological and environmental samples. The results obtained by LSC-B method give smaller error and larger 14C dating range limit, up to 60000 yr (the GPC dating limit is 37000 yr). Therefore, the LSC-B method is more suitable for 14C dating of archaeological samples.

14C; LSC; methods; benzene; direct absorption

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